1. Field of the Invention
The present invention relates to a vertical (vertical-electrode-type) GaN-based light emitting diode (LED) and a method of manufacturing the same, and more particularly, to a vertical GaN-based LED and a method of manufacturing the same, capable of increasing the external quantum efficiency and the manufacturing yield by optimizing the current diffusion effect and increasing the light-extraction efficiency.
2. Description of the Related Art
Generally, a GaN-based LED is grown on a sapphire substrate, but the sapphire substrate is a rigid nonconductor and has poor thermal conductivity. Therefore, there is a limitation in reducing the manufacturing costs by decreasing the size of a GaN-based LED, or improving the optical power and chip characteristic. Particularly, because application of a high current is essential for achieving high power LED, it is important to solve a heat-sink problem of the LED. To solve this problem, there has been proposed a vertical GaN-based LED in which a sapphire substrate is removed using a laser lift-off (LLO).
However, the conventional vertical GaN-based LED has a problem in that photon generated from an active layer is emitted to the outside of the LED. That is, an external quantum efficiency is degraded.
FIG. 1 is a graph for explaining the reduction of an external quantum efficiency in a conventional vertical GaN-based LED. Referring to FIG. 1, an incident angle θ1 at which photon is incident from a GaN layer to air should be less than a critical angle θc so that photon generated from an active layer can pass through the GaN layer having a refractive index N1 greater than a refractive index N2 of air and then escape into air.
When an escape angle θ2 at which the photon escapes into air is 90°, the critical angle θc is defined as θc=sin−1 (N2/N1). When light propagates from the GaN layer to air having a refractive index of 1, a critical angle is about 23.6°.
When the incident angle θ1 is greater than the critical angel θc, photon is totally reflected at an interface between the GaN layer and the air and goes back into the LED. Then, the photon is confined inside the LED, so that the external quantum efficiency is greatly reduced.
To prevent the reduction of the external quantum efficiency, U.S Patent Publication No. 20030222263 discloses that hemispherical convex patterns are formed on the surface of an n-type GaN layer to reduce an incident angle θ1 of a photo incident to air from the GaN layer below a critical angle θc.
A method for manufacturing a vertical GaN-based LED disclosed in U.S. Patent Publication No. 20030222263 will be described with reference to FIGS. 2 to 4.
FIGS. 2A to 2C are sectional views illustrating a method of manufacturing the vertical GaN-based LED disclosed in U.S. Patent Publication No. 20030222263, FIGS. 3A to 3C are enlarged sectional views illustrating a method of manufacturing the vertical GaN-based LED, and FIG. 4 is a sectional view of the vertical GaN-based LED manufactured through the method of FIGS. 2A to 2C and FIGS. 3A to 3C.
Referring to FIG. 2A, an LED structure 16 having GaN and a positive electrode (p-electrode) 18 are formed on a sapphire substrate 24, and a first Pd layer 26 and an In layer 28 are formed on the p-electrode 18. A second Pd layer is formed under a silicon substrate 20.
Referring to FIG. 2B, the silicon substrate 20 where the second Pd layer 30 is formed is attached to the p-electrode 18 where the first Pd layer 26 and the In layer 28 are formed.
Referring to FIG. 2C, the sapphire substrate 24 is removed using an LLO process.
Referring to FIG. 3A, photoresist patterns 32 are formed on predetermined portions of the surface of the exposed LED structure 16 (more specifically, the surface of the n-type GaN layer).
Referring to FIG. 3B, the photoresist patterns 32 are formed in a hemispherical shape through a re-flow process.
Referring to FIG. 3C, the surface of the LED structure 16 is etched through an anisotropic etching process so as to be patterned in a hemispherical shape.
Referring to FIG. 4, a negative electrode (n-electrode) 34 is formed on the LED structure 16. Through these procedures, the vertical GaN-based LED having the LED structure 16 whose surface is patterned is completed.
However, according to the vertical GaN-based LED manufactured by the method disclosed in U.S. Patent Publication No. 20030222263, when the LED structure 16 has a thickness of 10 μm or less (thin GaN), the process of forming the photoresist patterns 32 and subsequent processes are difficult to carry out even if the silicon substrate acting as a sub support is used. Accordingly, the manufacturing yield significantly decreases.
In addition, because the patterns for improving the external quantum efficiency are formed in a convex hemispherical shape on the surface of the LED structure, the surface of the LED structure on which the patterns can be formed is limited. Accordingly, the improvement of the external quantum efficiency that can be obtained by applying the convex hemispherical patterns is not sufficient. Therefore, there is a demand for a new method that can maximize the improvement of the external quantum efficiency.
Furthermore, because the surface of the LED structure (more specifically, the n-type GaN layer) contacting the n-electrode is formed by doping n-type conductive impurities (e.g., Si) into an undoped GaN layer, the LED structure has high doping concentration. Therefore, current crowding occurs only at a lower portion of the n-electrode contacting the surface of the LED structure and a current does not diffuse over an entire active layer. Consequently, the light-extraction efficiency of the LED is degraded and the lifespan of the LED is reduced.